Transmission line intruder detection system



March 26, 1963 M. E. TRIMBLE ETAL 3375,51I

TRANSMISSION LINE INTRUDER DETECTION SYSTEM Filed June 24, 1965 4 Sheets-Sheet 1 BALUN BALUN HYBRID I TRANSMITTER SIGNAL PROCESSER AND ALARM CIRCUIT TO BALUN l9 AND z-wms LINE 30 J 4 5 T0 BALUN 20 AND 2-WIRE LINE 4o INVENTORS MELVIN E. TRIMBLE CRAIG G ROBERTS BERNARD $.GOLOSMAN BY yaw %M/ ATTORNEY March 26, 1968 v M. E. TRIMBLE ETAL TRANSMISSION LINE INTRUDER DETECTION SYSTEM Filed June 24, 1965 4 Sheets-Sheet 5 INVENTORS MELVIN E. TRIMBLE CRAIG G. ROBE RTS BERNARD S. GOLOSMAN ATTORNEY 4 Sheets-Sheet 4 Mar h 6, 1968 M. E. TRIMBLE ETAL TRANSMISSION LINE INTRUDER DETECTION SYSTEM Filed June 24, 1965 N m M E O L S 8 T B T 0 N L E E V R B G NTO Y I RS E zany/w N R 0 I T V T LAR A RE MCB United States Patent 3,375,511 TRANSMISSION LINE INTRUDER DETECTION SYSTEM Melvin E. Trimble, San Jose, Craig G. Roberts, Cupertiuo, and Bernard S. Golosman, Paio Alto, Calif., assiguors to Sylvauia Electric Products Inc., a corporation of Delaware Filed June 24, 1965, Ser. No. 466,612 22 Claims. (Cl. 340258) This invention relates to intruder detection systems and more particularly to a perimeter type intruder detection system which bounds or surrounds an outdoor area to be protected.

In certain applications, it is necessary to prevent persons from entering prescribed areas. Physical barriers, such as Wire fences, are not always effective in discouraging such intruders from entering the area. It is therefore desirable to know when an intruder enters a protected area.

In the past, sophisticated techniques have been employed to determine when there has been an unauthorized intrusion of the protected area. One technique is to provide a corridor of radio frequency electromagnetic signals around the area to be protected. Reflected signals are processed to determine when an intruder penetrates that corridor. A major difliculty with such systems, however, is the tendency of the radio frequency energy to project beyond the corridor and therefore such systems are subject to false alarms caused by moving objects, such as persons or cars, in the vicinity of the protected area. Other fencetype intruder detection systems including pulse-type radar systems, Z-Wire capacitance type systems, and electrified chain link fences have been proposed but are susceptible to false alarms generated by small animals and birds and the elements (Wind, lightning, rain and snow) or may be too easily defeated or compromised. This invention is directed to the provision of a practicable intruder detection system which avoids these disadvantages.

An object of this invention is the provision of a fencetype intruder detection system which alarms in response to a human intruder but does not alarm when penetrated by small animals and birds.

Another object is the provision of a symmetrical perimeter presence detection system that is insensitive to fixed discontinuities (such as support posts) located at predetermined positions.

Another object is the provision of a perimeter presence detection system that is relatively insensitive to changes in ground conditions caused by rain or the like.

Another object is the provision of a fence-type detection system which cannot readily be intentionally .defeated or compromised.

Another object is the provision of a perimeter presence detection system employing standard commercially available material that is simple and economical to construct and relatively maintenance free.

These objects are accomplished by a system sensitive to reflections on a pair of balanced 2-wire transmission lines which define the boundary of the protected area and are symmetrical about the point at which electrical energy is fed to the lines. Each 2-Wire transmission line is terminated in its characteristic impedance and is balanced against the other line. A change in the impedance of one line caused by the proximity of an intruder to that line produces a reflection signal which is detected at the input to the line. This signal is processed in a receiver and produces an audible, visual or other alarm to indicate presence of the intruder. Transmission line conductors are rigid pipes and are vertically stacked on support posts at precisely spaced longitudinal intervals to insure equal spacing between adjacently stacked pipes. The conductors adjacent to the ground are housed in electrically nonconductive large diameter conduits which space groundroaming animals from the bottom conductors sufliciently to prevent alarming of the system thereby. The 2-wire transmission lines extend along the boundary of the protected area symmetrically from the input or feed point. The support posts are spaced a quarter wavelength apart at the operating frequency and symmetrically about the input so that the effect of discontinuities presented by the posts cancel. Similarly, expansion joints in the pipe condoctors are symmetrically located about the input for the same reason, 1'.e., the discontinuities are common to each transmission line and the effects thereof cancel. A synthetic ground plane of electrically conductive wire is located under the transmission lines to compensate for changes in ground conditions, such as may be caused by rain, and to reduce noise picked up by the transmission lines.

As a countermeasure against efforts to intentionally defeat the system by a balanced intrusion, i.e., two intruders simultaneously approaching the respective transmission lines at points equally spaced from the input, barrier conductors or pipes are mounted parallel to and between adjacent conductors at zero potential points. The barrier pipes extend across alternate spaces between the posts and at different distances on the respective line from the input point so that the barrier pipes are asymmetrical about the input. The barrier pipes thus present a physical barrier across alternate spaces between support posts.

This invention and these and other objects thereof will be more fully understood from the following description in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic diagram of an intruder detection system embodying this invention;

FIGURE 2 is a detailed schematic and block diagram of the system of FIGURE 1 including two transmission line systems;

FIGURE 3 is a schematic diagram of a circuit for terminating the two conductors of one of the transmission lines;

FIGURE 4 is a schematic diagram of a coaxial hybrid signal branching network used as one of the system components;

FIGURE 5 is a fragmentary schematic view of a pair of baluns and input connections to the conductors comprising a pair of Z-Wire lines;

FIGURE 6 is an enlarged perspective view of one of the transmission lines shown in FIGURE 2;

FIGURE 7 is an enlarged elevation of a pipe conductor expansion joint;

FIGURE 8 is a perspective view, partially in section, of a modified form of 2-wire transmission line;

FIGURE 9 are waveforms illustrating operation of the system;

FIGURE 10 is a perspective view of a modified form of 2-wire transmission line;

FIGURE 11A is a diagrammatic transverse sectional view of a pair of laterally spaced parallel 2-wire lines and illustrating the field pattern around the conductors; and

FIGURE 11B is a similar view of a pair of 2-wire lines employing a common lower conductor.

Referring to FIGURE 1, an intruder detection system embodying the invention comprises a transmitter 1, a signal processor and alarm circuit 2, and two 2-wire transmission lines 3 and 4 which surround the area A to be-- protected. The electronic circuitry indicated at 1 and 2 is shown outside the protected area for simplicity. In a preferred embodiment of the invention shown in FIGURE 2, two 2-wire transmission lines 3a and 3b are vertically stacked in the form of a fence of predetermined height,

and similarly transmission line 4 comprises stacked 2-wire lines 4a and 4b. The transmission lines extend between inputs at I and terminations at T and are symmetrical about a vertical plane containing the axis XX through the input. The conductors of the transmission lines are supported by posts P spaced apart by a quarter wavelength at the center of the transmitter operating frequency.

Each 2-wire line is terminated at its end T by a load 5, see FIGURES 3 and 6, comprising a resistor 6 and capacitor 7 electrically connected in series between the conductors of the Z-wire line. The resistance of resistor 6 is substantially equal to the characteristic impedance of the 2- wire line to provide a substantially reflectionless load. The capacitance of capacitor 7 is selected to produce a series resonance condition in combination with the inductance of vertical leads 8 and 9 so as to reduce pick-up of noise by the vertical leads.

Transmitter 1 comprises continuous wave radio frequency oscillators 10' and 11, see FIGURE 2, and a modulator 12. The output of oscillator 10 may be a 30 mc.p.s. sinusoidal signal that is modulated at a l kc.p.s. rate by the sinusoidal output of oscillator 11. The output of modulator 12 is applied on lines 13 and 14 to hybrid signal branching networks 15 and 16, respectively. The lines, including lines 13 and 14, which connect components of the system preferably are coaxial transmission lines. Since the outer conductors of these coaxial transmission lines are connected to a ground reference potential, they are omitted from the drawings for simplicity of illustration and only the center conductors are shown. The outputs of hybrid 15 on lines 17 and 18 are coupled through baluns 19 and 20 to ?.-wire lines 3a and 4a, respectively. Similarly, the outputs of hybrid 16 on lines 21 and 22 are coupled through baluus 23 and 24 to 2-wire lines 3b and 412, respectively.

Hybrids 15 and 16 are substantially identical and may, by way of example, be coaxial hybrids ANZAC, Model H, manufactured by Anzac Electronics Incorporated, see FIGURE 4. Since the hybrids are alike, only the one hybrid 15 will be described. Hybrid 15 comprises a transformer having primary windings 27 and 28 connected in parallel. One terminal of each primary winding is connected to ground. The other terminals of the primary windings are each electrically connected to line 13. The tap 29 of the center tapped secondary, comprising Windings 30 and 31, is connected to line 25 which is the output of hybrid 15. The other terminals of windings 30 and 31 are connected through lines 17 and 18, respectively, to the associated 2-wire lines 3a and 4a. The outer conductors of coaxial lines 17, 18 and 25 are connected to ground at 33.

Consider that a signal V having the polarity indicated in FIGURE 4, is applied to hybrid 15 between line 13 and ground. Since primary windings 27 and 28 are identical and are connected in parallel, the signals developed across the windings are in phase. The signals V and V coupled across secondary windings 30 and 31 are also equal in magnitude. Since the secondary windings are floating with respect to ground, however, the signals V and V are 180 out of phase and tap 29 is effectively at ground potential. When the transmission lines are balanced such that the characteristic impedances of 2-wire lines 3a and 4a transformed between line 17 and ground and line 18 and ground, respectively, are equal, the signals V and V developed between line 17 and ground and line 18 and ground, respectively, are of equal mag nitude and 180 out of phase. The signals V and V and the signals V and V are therefore equal and the hybrid is balanced such that there is no output of hybrid 15 on line 25. When an intruder or a discontinuity is present on only one transmission line, however, the characteristic impedances transformed to and signals developed between line 17 and ground and line 18 and ground are not equal. The hybrid is therefore unbalanced, since the signals V and V and the signals V and V are not equal. An out- 4 put of hybrid 15 is generated on line 25 that is processed and an alarm is actuated to indicate that an intriuder is proximate to the transmission lines.

The baluns 19, 20, 23 and 24' provide the proper impedance transformation between the associated hybrids and the Z-wire lines. The baluns may be coaxial balunsof the type illustrated in FIGURES. Since baluns 19 and 20 and baluns 23 and 24 are connected to the associated components in the same manner, only the connections of baluns 19 and 20 are described below and are shown in 1 FIGURE 5.

Balun 19 comprises a coaxial transmission line 36 that is a half wavelength long at the operating frequency of oscillator 10. The outer conductorsof coaxial lines 17 and 36 are electric-ally connected to ground. One end of the center conductors of coaxial lines 17 and 36 are connected together at 37 and through lead 38 to upper conductor 39 of 2-wire line 3a. The other end 40 of the center conductor of coaxial line 36 is connected through lead 41 to lower conductor 42 of 2-wire line 3a. Balun 20 also comprises a coaxial transmission line 36a that is a half wavelength long at the operating frequency of oscillator 10. The ends of the center conductors of coaxial lines 18 and 36a that are connected at 37a, however, are connected through. lead 43 to the lower conductor 44 of Z-Wireline 4a. The other end 40a of the center conductor of coaxial line 36a is connected through lead 45 to the upper conductor 46 of 2-wire line 4a.

Since the opposite ends of the center conductors of coaxial lines 36 and 36a are separated by a half wave length, the electrical signals induced in lines 37 and 40 (and 37a and 40a) are 180 out of phase as indicated by the arrows 47, 48 (and 47a and 48a). The curents induced in the adjacent vertical leads 38 and 45 and leads 41 and 43 connected to theupper conductors and the lower conductors, respectively, of lines 3a and 4a are therefore 180 out of phase as indicated by the associated arrows.

The output of hybrid 15 on line 25 is filtered by band-1 pass filter 51, see FIGURE 2, and is rectified by detector 52. to provide an audio signal. Similarly, the output of hybrid 16 on line 26 is filtered by bandpass filter 53 and is rectified by detector 54.'The outputs. of detectors 52 and 54 are combined in bandpass amplifier 55 which is tuned to amplify the 1 kc.p.s modulation signal. The

amplified signal is applied to synchronous detector 56. The 1 kc.-p.s. modulation signal generated by oscillator 11 is a reference signal aplied on line 57 to the. synchronous detector. The output of detector 57. is threshold detected by Schmitt trigger 58 and is summed by integrator 59. The signal stored by integrator 59 is monitored 1 by Schmitt trigger 60 which controls the operation of an alarm indicator 61.

In operation, the continuous wave radio frequency (RF) signal from oscillator 10 is modulated by the output of oscillator 11. The modulated signal is divided by hybrids 15 and 16 and is applied to the 2-wire lineswhich define the perimeter of the protected area. Since each line is terminated in its characteristicimpedance, the modulated RF signal is not reflected from the line terminations at T to the inputs at I and a signal is not applied on lines 25 and 26 to the signal processor and alarm circuits. Consider that an intruder approaches transmission line 3. The presence of the intruder adjacent to the transmission line causes the characteristic impedance of the line to change and presents discontinuities on the 2-wire lines 3a and 3b. A portion of the incident RF signal is refiected by the discontinuity and is coupled to the hybrids. The change .in characteristic impedance of2-wire lines 3a and 3b also produces an imbalance of hybrids 15 and 16. Output signals from the hybrids therefore appear on lines 25 and 26. These outputsfrom the hybrids, after being filtered and detected, are summed by amplifier 55.. The amplified signal is synchronous detected to pass signal variations in phase with the 1 .kc.p.s. modulation signal J and discriminate against extraneous signals. When the output of the synchronous detector exceeds the threshold level established by Schmitt trigger 58, a signal is summed by integrator 59. When the signal stored by integrator 59 exceeds the threshold level established by Schmitt trigger 60, alarm indicator 61 is actuated to indicate an intrusion of the protected area.

The transmission lines of the apparatus are exposed to the elements and must therefore remain insensitive to wind, temperature, rain, snow, lightning and the like. Particularly, since the system operates on the reflection principle and characteristically is highly sensitive, the conductors of the transmision lines must be sufliciently rigid to restrict vertical movement thereof due to wind so as to maintain equal spacing of the conductors. Also, the conductors must be able to longitudinally expand and contract without buckling when the temperature changes. Such requisite rigidity of the conductors is obtained in the preferred embodiment of the invention by use of electrically conductive pipe, such as one inch aluminum pipe. These pipe conductors are firmly supported by the wood posts, and may extend through support openings 62 in the posts as shown in FIGURE 6. The conductors are rigidly secured to the posts at the input I as by nails 63 extending through the posts and pipe conductors.

Thermal expansion and contraction of the pipe conductors is accommodated by relatively short expansion joints 64, see FIGURES 6 and 7, in the conductors. Each expansion joint comprises a braided conductive sheet 65 electrically connected adjacent ends of axially spaced pipe conductors 66a and 66b. Conductive sheet 65 is wrapped around conductors 66a and 66b and is secured thereto by clamps 67 and 68. Braided sheet 65 is sufficiently flexible to readily elongate or expand as required by movement of the associated pipe conductors. The pipe conductors are maintained level at the expansion joint by a tubular insert 69 extending into pipe conductors 66a and 66b. Tubular insert 69 may be wood doweling or metal pipe that is secured to one of the conductors.

The support posts are preferably made from finished grade lumber. In order to prevent coagulation of water in droplets on the posts and thus avoid resultant discontinuities on the transmission lines, the posts are provided with a smooth surface through application of a silicon varnish and an exterior enamel paint.

The reliability of intrusion detection systems is directly related to their false alarm rates, i.e., the number of alarms generated by the system over a given period of time when there were in fact no intrusions by humans. Outdoor systems to which this invention is directed are therefore required to discriminate against roaming animals which might produce such a false alarm. A small animal, such as a rabbit, closely spaced to or actually contacting a pipe conductor may well generate a reflected signal of the same magnitude as one caused by a human intruder spaced a greater distance from the conductor and thus alarm the system. It is therefore important that such animals most likely to produce false alarms be prevented from approaching the system transmission line conductors sufficiently closely to have an adverse effect. Small animals are an especially troublesome source of false alarms, and since such animals typically affect only the bottom conductor of the stacked group, provision is made to insure safe spacing of small animals from the lowermost pipe conductors.

In accordance with this invention, a minimum spacing between animals and the lower pipe conductor 71 is maintained by a nonconductive protective pipe 72 enclosing the conductor. Pipe 72 is preferably made of a bituminous fiber dielectric material and may be Orangeburg Rigid Conduit, manufactured by Orangeburg Manufacturing Company. Pipe conductor 71 is supported in and coaxial with protective pipe 72 by nonconductive disks 73. Protective pipe 72 is supported at each post by a bracket 74 and between each post by a ground-mounted 6 dielectric support 75. Alternatively, all conductors of the transmission lines may be enclosed in protective pipes as illustrated in FIGURE 8.

Since a bend in a transmission line presents a discontinuity on the line, it is desirable that a transmission line be straight. It is necessary in most instances, however, to shape the transmission lines to completely enclose the area to be protected. If the transmission line is made with gradual bends or large radius curves in order to minimize discontinuities, lines considerably longer than the perimeter of the protected area are required.

We have discovered that the discontinuity presented by an abrupt bend, such as a right angle bend, in the transmission line is similar to the discontinuity presented by a capacitor connected between the conductors of a 2- wire line. In accordance with this invention, therefore, sharp bends are employed to shape the Z-Wire lines around the protected area. The discontinuity presented by the bend is compensated by an inductor 77, see FIG- URE 6, connected between the conductors of the 2-wire line at the bend. Inductor 77 and the capacitor (representing the effect of the bend) are shunt resonant to provide an open circuit between the conductors and thereby compensate for the effects of the discontinuity presented by the bend.

The magnitude of reflected signals on transmission lines of the above-described intruder detection system is relatively small. It is therefore desirable that noise in and radiation loss of the system be minimum. We have discovered that atmospheric noise picked up by the transmission lines is reduced by a conductive ground plane 79, see FIGURES 2 and 6, below the transmission lines. The conductive ground plane is maintained at the ground reference potential. In a system that was constructed and successfully operated, the conductive ground plane employed was common chicken wire covered with a thin layer of dirt.

Vertical leads 8 and 9 of load 5, see FIGURE 3, have an inherent inductance and therefore operate as antenna elements. The inductance of leads 8 and 9 is series resonant with capacitor 7, however, to reduce the effect of the leads. Thus, noise picked up by the vertical leads is reduced.

The vertical leads connecting the associated baluns and conductors of the 2-wire lines also have an associated inductance and therefore operate as antenna elements, see FIGURE 5. Since the currents in adjacent vertical leads (leads 38 and 45 and leads 41 and 43) are equal and are out of phase, the fields associated With the adjacent leads are also equal and 180 out of phase and therefore cancel to reduce radiation from and noise picked up by the leads.

It is desirable in a null or reflection type system that there be no discontinuities or sources of reflection associated with the transmission lines. It is difficult in practice, however, to make an intruder detection system with reflectionless transmission lines. For example, it is necessary to support the conductors of the transmission lines. It is also necessary to provide expansion joints to compensate for thermal effects of the conductors. In practice, each support post and expansion joint is a discontinuity and source of reflection of the transmission lines. It is desirable therefore to compensate for such discontinuities.

In accordance with this invention, the effects of discontinuities caused by posts supporting the conductors of the transmission lines are substantially reduced by axially locating support posts a quarter Wavelength apart. Each support post is spaced a quarter Wavelength, at the center frequency of oscillator 10, from adjacent support posts. Specifically, post 81 is axially spaced a quarter wavelength from adjacent posts 82 and 83, see FIGURE 6. Since the support posts are spaced a quarter wave length apart, the lengths of transmission line between adjacent posts 81 and 83 are impedance inverters that transform the impedances Z at one post 81 to the impedances where Z is the characteristic impedance of the lengths of transmission line between the posts 81 and 83. The impedance on the transmission line at each post is similarly transformed back to the input. The transformed impedances add at the input such that the resultant impedance is substantially equal to the characteristic impedance of the transmission line. The discontinuitles presented by the support posts therefore cancel and are effectively removed from the transmission lines. This cancellation is illustrated graphically in FIGURE 9.

The posts P, see FIGURE 9, are spaced a quarter wavelength apart at the operating frequency of oscillator 10. The standing wave S is induced on the transmission line and is supported by the output of oscillator 10. The magnitude M of the standing wave S at input post P is not equal to the magnitude M of the standing Wave S a quarter wavelength away at adjacent post P The absolute magnitude [M is, however, equal to the absolute magnitude 1M of the standing wave S a half wavelength away at post P Reference to FIGURE 9 reveals that the magnitudes of the standing wave S at alternate posts (e.g., P and P tervals) are of equal magnitude and opposite sign.

Post P is physically spaced a quarter wavelength from input post P Since an electromagnetic signal travels a quarter wavelength to post P and a reflected signal travels another quarter Wavelength back to input post P post P is effectively spaced two quarter wavelengths or a half wavelength from input post P Each post is therefore effectively spaced an even number of quarter wavelengths (or an integral number of half wavelengths) from input post P Since post P is effectively spaced a half wavelength from input post P post P effectively folds or transforms the standing wave at post P back to input post P as represented by the dotted waveform S The magnitude M, of the standing wave S and the magnitude M of the reflected standing wave S are equal and of opposite sign and therefore cancel.

Other posts similarly fold the standing wave at an associated post (e.g., post P is associated with post P back to input post P The transmission line employs an even number of posts (including input post P and thereby provides maximum cancellation of the effects of the discontinuities presented by the posts.

Reflections caused by discontinuities such as expansion joints are substantially reduced by (1) employing a pair of transmission lines 3 and 4 that are symmetrical about the reference line XX through the input I, see FIG- URE 2, (2) employing hybrids and 16 having a 180 phase difference associated with coupled outputs 17 and 18, and 21 and 22, respectively, see FIGURES 2 and 4, and (3) locating discontinuities so that two identical discontinuities a-re spaced an integral number of quarter wavelengths apart on a single transmission line or so that there are identical discontinuities at the same relative points (with respect to input I) on each transmission line. Referring to FIGURE 2, the expansion joints indicated at 86 and 87 on transmission line 3 and at 86 and 87' on transmission line 4 are positioned four quarter wavelengths apart. The discontinuities presented by the expansion joints therefore cancel in the manner described above in relation to the cancellation of the discontinuities caused by the support posts.

The expansion joints indicated at 86 and 86' (and also at 87 and 87) are identical and are located at the same relative points (with respect to input I) on transmission lines 3 and 4, respectively. The signals reflected by the expansion joints (and also the impedances transformed back to the inputs of the 2-wire lines) are therefore the same magnitude. The signals V and V between line 17 and ground and line 18 and ground, see FIGURE 4, are therefore equal and 180 out of phase. Thus, the signals at half wavelength in- 8 V and V and the signals V and V are equal so that the hybrid is balanced. The effects of identical discontinuities (at 86 and 86'), at the same relative point on each transmission line, therefore cancel and do not cause an output to be generated by the hybrids.

The most vulnerable points of the system, i.e., the points at which the lines are least sensitive to intruders, are at the minimum potential points between the conductors of the 2-wire lines. In other words, an intruder at one of these points presents a minimum discontinuity on a 2-wire line. The transmission lines illustrated. in FIG- URE 10 obstruct a balanced intrusion by strategic-ally located intruders passing through the transmission lines at the minimum potential points by means of physical barriers located at periodic intervals along the minimum potential points. The barriers are asymmetrical about input I, i.e., the barriers between lines 41: are spaced differently from the input point I than the barriers between lines 3a. In order to simplify the drawings, connections at the input I are omitted from the drawing.

Barrier pipes 91a, 9111, etc. are coincident with the zero potential line between the conductors of 2-wire line 3a. Barrier pipes 92a, 92b, etc. are coincident with the zero potential line between the conductors of 2-wire line 4a. Barrier pipe 91a .is secured to and extends between sup-port posts 93 and 94 and is adjacent the input I.Barrier pipe 92a, however, is secured to and extends between support posts 95 and 96 and is not adjacent the input I. Thus, pipes 91a and 9211 are asymmetrical about input I. The other pipes 9117, etc. and 921), etc. are also asymmetrically located about the input I. Each barrier pipe is secured to and extends between alternate pairs of support posts. Thus, alternate ones of the spaces between adjacent support posts are traversed by a barrier pipe 91 or 92. Barrier pipes 97a, 9712, etc. and 98a, 9812, etc. are similarly located between the conductors of 2-wire lines 3b and 4b, respectively. Barrier pipe 98a is mounted adjacent input I. Barrier pipes are not required between adjacent conductors 101 and 102 of transmission line 3 and adjacent conductors 103 and 104 of transmission line 4 because a strong electromagnetic field exists therebetween.

When two intruders located at the same relative point on both sides of the input or spaced a quarter wavelength apart approach the minimum potential point of a 2-wire line, one of the intruders is confronted with a physical barrier extending between adjacent support :posts. Since the physical barrier (e.g., barrier pipe 91a) is located at the minimum potential point between the pipe conductors of the associated 2-wire line, the intruder is forced to go over or under the pipe and penetrate the line at a more sensitive point. In an actual system, such a barrier made it virtually impossible for intruders to compromise the apparatus and enter the protected area undetected.

A modified transmission line for compensating for effects caused by snow, see FIGURE 11A, comprises a pair of parallel 2-wire lines 106-and 107 that are symmetrical with respect to the input. Upper conductors 108 and 109 are located the same distance above the ground. The lower conductors 110 and 111 are also located the same distance above ground and are enclosed in protective pipes 112. The conductors of the 2-wire lines may be energized as illustrated in FIGURE 11A. Alternatively,

the 2-wire lines may employ a common lower conductor 113 and may be energized as illustrated in FIGURE 11B. In an actual embodiment, lower conductor 113 and the upper conductors 108" and 109' were located one foot and three and one-half feet above the ground, respectively. Conductors 108' and 109' were spaced two feet apart.

During 'a snow storm, there is normally a slow and even build-up of snow on the ground. When the snow is blown by wind, however, it concentrates in drifts. The slope of the surfaces of these snow drifts normally varies gradually over a considerable distance. Under suchconditions, the depth of snow below the conductors of the 2-wire lines is approximately. constant. The effects of discontinuity presented on the 2-wire lines by the snow are therefore approximately equal and cancel in the manner described previously in relation to identical discontinuities at the same relative point on the transmission lines. In an actual apparatus, the slope 'of snow under the conductors of the 2-wire lines was greater than 10 before the apparatus false-alarmed.

Although this invention is described in relation to a preferred embodiment thereof, variations and modifications will be apparent to those skilled in the art. For example, the signal branching networks may comprise ferrite circulators, directional couplers, or other similar devices. Also, the pipe conductors may be secured to the support posts by dielectric or metallic straps or clamps. The pipe conductors of the 2-wire lines may be supported by dielectric or metal posts. When electrically conductive support posts are employed, the pipeconductors are electrically connected thereto through a quarter wavelength section of transmission line to present an open circuit between the conductors. Also, one of the transmission lines in FIGURE 1 may be completely replaced by an impedance having a value equal to the characteristic im pedance of the other transmission line. Thus, both baluns are terminated in their characteristic impedances and the system is balanced. The scope and breadth of this invention is there-fore to be determined from the following claims rather from the above detailed description of a preferred embodiment thereof.

What is claimed is:

1. In a transmission line perimeter intruder detection system, a transmission line comprising a pair of stacked rigid conductors bounding an area to be protected,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at a predetermined frequency,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent support posts and extending between alternate spaces between said posts, and

an impedance connected between said conductors at one end thereof, said impedance having a value substantially equal to the characteristic impedance of said transmission line.

2. The transmission line according to claim 1 including an electrically nonconducting member surrounding at least one of said stacked conductors.

3. The transmission line according to claim 1 including an electrically nonconducting member surrounding the lower one of said stacked conductors over a substantial length of said lower conductor.

4. The transmission line according to claim 3 including an electrically conductive ground plane under said lower conductor and electrically connected to a reference potential.

5. In a perimeter intruder detection system including a transmission line bounding an area to be protected, a transmitter for transmitting a radio frequency signal having a predetermined frequency on said transmission line, and a detector responsive to reflected signals on said transmission line for determining and indicating intrusion of the protected area, a transmission line comprising a pair of stacked rigid conductors,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at the predetermined frequency,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent support posts and extending between alternate spaces between said posts, and

an impedance connected between said conductors at one end thereof, said impedance having a value substantially equal to the characteristic impedance of said transmission line.

6. The perimeter intruder detection system according to claim 5 including an electrically nonconducting member surrounding the lower one of said stacked conductors.

7. A perimeter intruder detection system comprising a transmission line comprising a pair of stacked rigid conductors bounding an area to be protected,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at a predetermined frequency,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between said posts, and

a first impedance connected between said conductors at one end thereof, said impedance having a value substantially equal to the characteristic impedance of said transmission line,

a second impedance having a value substantially equal to the characteristic impedance of said transmission line,

a transmitter having an output connected to said second impedance and to the other end of said conductors, said transmitter generating a radio frequency signal having the predetermined frequency,

means for coupling from said transmission line a signal reflected to said other end of said conductors, and

a detector responsive to the output of said coupling means for determining and indicating intrusion of the protected area.

8. A perimeter intruder detection system comprising a transmitter generating a radio frequency signal having a predetermined frequency,

a transmission line comprising a pair of stacked rigid conductors bounding an area to be protected,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at the predetermined frequency,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between said posts,

'an electrically nonconductive member surrounding one of said conductors, and

a first impedance connected between said conductors at one end thereof, said impedance having a value substantially equal to the characteristic impedance of said transmission line,

a second impedance having a value substantially equal to the characteristic impedance of said transmission line,

a signal branching network having an input responsive to the output of said transmitter, having a first output connected to the other ends of said conductors, having a second output connected to said second impedance for balancing said network and having a third output, said network coupling the output of said transmitter to said conductors and coupling to said third output the signal reflected on said conductors to said first output, and

a detector responsive to the reflected signal at said third output of said signal branching network for determining and indicating intrusion of the protected area.

9. In a transmission line perimeter intruder detector, a

transmission line system comprising a first transmission line comprising a first pair of stacked rigid conductors bounding an area to be protected, said first conductors having an input end,

a second transmission line comprising a second pair of stacked rigid conductors bounding the area to be protected, said second conductors having an input end adjacent said input end of said first conductors,

said transmission lines being electrically symmetrical with respect -to said input ends,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at a predetermined frequency and being symmetrically located with respect to said input ends,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between said posts such that said barrier members are asymmetrical with respect to said input ends,

a first impedance connected between said first conductors at the other end thereof, said first impedance having a value substantially equal to the characteristic impedance of said first transmission line, and

a second impedance connected between second conductors at the other end thereof, said second impedance having a value substantially equal to the characteristic impedance of said second transmission line.

10. The transmission lines according to claim 9 including an electrically nonconductive member surrounding at least one of said conductors.

11. A perimeter intruder detection system comprising a first transmission line comprising a first pair of stacked rigid conductors bounding an area to be protected, said first conductors having an input end,

a second transmission line comprising a second pair of stacked rigid conductors bounding the area to be protected, said second conductors having an input end adjacent said input end of said first conductors,

said transmission lines being symmetrical about a line through the mid-point between said input ends,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at a predetermined frequency and being symmetrically located about said line,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between said posts such that said barrier members are asymmetrical about said line,

a first impedance connected between said first conductors at the other end thereof, said first impedance having a value substantially equal to the characteristic impedance of said first transmission line,

a second impedance connected between said second conductors at the other end thereof, said second impedance having a value substantially equal to the characteristic impedance of said second transmission line,

a transmitter having an output electrically connected to said input ends of said conductors of said first and second transmission lines, said transmitter generating a radio frequency signal having the predetermined frequency,

means for coupling from said transmission lines signals reflected to said input ends of said conductors by an intruder adjacent one of said transmission lines, and

a signal processing circuit responsive to the output of said coupling means for determining and indicating intrusion of the protected area.

12. The intruder detection system according to claim 11 including electrically nonconductive members surrounding the lower conductor of each of said pairs of conductors.

13. The intruder detection system according to claim 11 wherein said impedances each comprise a resistor having a resistance substantially equal to the characteristic impedance of the associated transmission line, and

a capacitor having a capacitance that is series resonant with the inductance of the vertical leads connecting the elements of said impedances to the associated conductors.

14. The intruder detection system according to claim 11 wherein said transmitter comprises a first radio frequency oscillator oscillating at the predetermined frequency,

a second radio frequency oscillator oscillating at a frequency less than the predetermined frequency, and

a modulator responsive to the outputs of said first and second oscillators for generating a modulated radio frequency signal.

15. The intruder detection system according to claim 11 wherein said signal processing circuit comprises a synchronous detector having a first input responsive to the output of said coupling means and having a second input responsive to the output of said second oscillator, said synchronous detector rectifying the output of said coupling means when that output is in phase with the output a of said second oscillator.

16. A perimeter intruder detection system comprising first and second transmission lines enclosing an area to be protected, each of said transmission lines comprising a pair of stacked rigid conductors, said conductors having adjacent input ends,

said transmission lines being symmetrical abouta line through the mid-point between said input ends,

an inductor electrically connected between one of said pairs of conductors at a bend in the associated transmission line, the inductance of said inductor being shunt resonant with the equivalent capacitance presented by said bend to present an open circuit between said conductors,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at, a predetermined frequency and being symmetrical about said line,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between posts such that said barrier members are asymmetrical about said line,

an electrically nonconductive member surrounding the lower conductors of each of said pairs of conductors,

an electrically conductive ground plane under said lower conductors and electrically connected to a reference potential,

first and second impedances connected between the other ends of said first and second pairs of conductors respectively, said impedances having values equal to the respective characteristic impedances of of the associated transmission lines,

a transmitter generating a radio frequency signal having the; predetermined frequency,

first and second baluns having inputs and having outputs connected at said input ends of said conductors of said first and second transmission lines, respectively,

a signal branching network having a first terminal connected to the input of said first balun, having a l second terminal connected to the input of said second balun, having a third terminal connected to the outi put of said transmitter for applying the radio frequency signal to said transmission lines, and having a fourth terminal passing signals reflected to said,

first or second terminal from a discontinuity on the associated transmission lines caused by an intruder adjacent thereto, and

a signal processing circuitresponsive to the output of said fourth terminal for determining and indicating the presence of an intruder adjacent one of said transmission lines.

17. The intruder detection system according to claim 16 including a plurality of expansion joints in said conductors, said expansion joints being symmetrically located about said line.

18. The intruder detection system according to claim 16 including a plurality of expansion joints in said con- 13 ductors, said expansion joints being spaced a quarter Wavelength apart at the predetermined frequency.

19. In a transmission line perimeter intruder detector, a transmission line system comprising first and second juxtaposed transmission lines bounding an area to be protected and having adjacent input ends and termination ends, each of said transmission lines being symmetrical with respect to said input ends and comprising a pair of stacked rigid conductors,

a plurality of posts supporting said conductors, said posts being spaced a quarter wavelength apart at a predetermined frequency and being symmetrical with respect to said input ends,

a plurality of barrier members located between said conductors, said barrier members being secured to adjacent posts and extending between alternate spaces between said posts,

an electrically nonconductive member surrounding the lower ones of each of said pairs of stacked conductors,

a first impedance connected between said conductors of said first transmission line at said termination end, said first impedance having a value substantially equal to the characteristic impedance of said first transmission line, and

a second impedance connected between said conductors of said second transmission line at said termination end, said second impedance having a value substantially equal to the characteristic impedance of said second transmission line.

20. The transmission line system according to claim 19 wherein said first and second transmission lines have a common lower conductor.

21. An intruder detection system comprising a pair of transmission lines having characteristic impedances and extending above a ground base along the boundary of an area to be protected, each of said transmission lines comprising a pair of rigid vertically stacked conductors having having predetermined diameters,

an impedance element connecting an end of one conductor to the adjacent end of the other conductor, and

a dielectric member enclosing the conductor proximate to the ground,

said member having a diameter substantially greater than the diameter of the conductor,

support means at predetermined longitudinal intervals along said lines for supporting the conductors of each line with equal vertical spacing,

a source of radio frequency energy applied to the ends of said conductors opposite from said impedance elements, and

detection means responsive to reflection signals on the conductors of both lines for indicating a change of impedance of one line relative to the other.

22. In a radio frequency transmission line outdoor perimeter intruder detection system energized to operate at a predetermined frequency above the ground, a trans- 15 mission line comprising a pair of vertically stacked rigid conductors bounding an area to be protected and having predetermined diameters,

a plurality of posts supporting said conductors with equal inter-conductor spacing throughout the length of the conductors, adjacent posts being longitudinally spaced apart by a quarter wavelength at said predetermined frequency,

a dielectric member supported on said posts and coextensive with and enclosing the conductor proxi mate to the ground, said member having a diameter substantially greater than the diameter of said conductor, and

an impedance element connected between said con- 30 ductors at one end thereof, said element having an impedance value substantially equal to the character istic impedance of said transmission line.

References Cited UNITED STATES PATENTS 2,424,677 7/1947 Brownlee 340258 2,492,388 12/1949 Martin 340258 X 2,956,269 10/1960 Schmidt 340258 3,047,849 7/1962 Hansen 340-258 3,230,518 l/1966 Vassil et al. 340-258 JOHN W. CALDWELL, Primary Examiner.

D. L. TRAFTON, Assistant Examiner. 

1. IN A TRANSMISSION LINE PERIMETER INTRUDER DETECTION SYSTEM, A TRANSMISSION LINE COMPRISING A PAIR OF STACKED RIGID CONDUCTORS BOUNDING AN AREA TO BE PROTECTED, A PLURALITY OF POSTS SUPPORTING SAID CONDUCTORS, SAID POSTS BEINS SPACED A QUARTER WAVELENGTH APART AT A PREDETERMINED FREQUENCY, A PLURALITY OF BARRIER MEMBERS LOATED BETWEEN SAID CONDUCTORS, SAID BARRIER MEMBERS BEING SECURED TO ADJACENT SUPPORT POSTS AND EXTENDING BETWEEN ALTERNATE SPACES BETWEEN SAID POSTS, AND AN IMPEDANCE CONNECTED BETWEEN SAID CONDUCTORS AT ONE END THEREOF, SAID IMPEDANCE HAVING A VALUE SUBSTANTIALLY EQUAL TO THE CHARACTERISTIC IMPEDANCE OF SAID TRANSMISSION LINE. 